Field of the Invention
This invention relates to a drive train of a vehicle, such as a mobile machine, with an internal combustion engine and working hydraulics driven by the internal combustion engine. The working hydraulics have at least one hydraulic pump driven by the internal combustion engine, which hydraulic pump can be operated as a pump and as a motor. When operated as a pump, the hydraulic pump takes in hydraulic fluid from a tank on a suction side and delivers into a delivery line that leads to the working hydraulics. When operated as a motor, the hydraulic pump can be supplied with hydraulic fluid from a hydraulic accumulator on the suction side. The drive train has a charge pump driven by the internal combustion engine to supply a charging circuit.
Description of Related Art
Self-propelled mobile machines, such as industrial trucks, agricultural equipment, forestry equipment, and construction equipment, such as excavators, wheeled and telescoping loaders, tractors, combine harvesters, forage harvesters, sugar beet and potato diggers, have a drive train with an internal combustion engine which drives a traction drive and the working hydraulics that perform the work functions of the machine. At least one hydraulic pump driven by the internal combustion engine is provided to supply the working hydraulics with hydraulic fluid.
During idle operation, when the traction drive and the working hydraulics are not actuated and, therefore, no torque is required from the internal combustion engine, the internal combustion engine is operated at a lower idle speed. Idle operation of this type occurs during pauses or interruptions in the work.
To reduce the fuel consumption of the internal combustion engine during pauses or interruptions in work, a start-stop function can be provided for the internal combustion engine in which the idling internal combustion engine is shut off during pauses or interruptions in work and is automatically restarted when there is a demand for torque from a work function or the traction drive. The shutoff and subsequent restarting of the internal combustion engine can occur even after relatively brief idle times, so that the starting process of the internal combustion engine is an operation that must be carried out correspondingly frequently and at short intervals during the operation of the internal combustion engine. This requirement places severe demands on the starter device of the internal combustion engine with regard to fatigue strength and the ability to deliver the starting energy required to start the internal combustion engine.
On internal combustion engines, such as diesel or gasoline engines, starter devices driven by an electric motor are generally used. To start the internal combustion engine from a shutdown, the speed of rotation necessary for the self-sustaining running of the internal combustion engine is produced with an electric starter motor connected with the crankshaft of the internal combustion engine by a transmission. The transmission is generally formed by a pinion gear on the output shaft of the electric starter motor and a ring gear on the crankshaft of the internal combustion engine and has a high translation ratio so that a high-speed and compact electric starter motor can be used.
On known starter devices operated with electric motors with a high-speed and compact electric starter motor, very high currents are required to flow for a short period of time to produce the torque necessary to start the internal combustion engine. The currents that are generated to start the internal combustion engine result in a significant increase in temperature. If the internal combustion engine has to be restarted after short intervals for a start-stop function, this operating behavior results in the overheating of known starter devices, the failure of the electric starter motor, and failure of the starter device operated by the electric motor. To be able to actuate a conventional starter device of this type driven by an electric motor with an electrical starter motor for a start-stop function at brief intervals, the level of the electric voltage must be increased and the electric starter motor must be designed so that it has a correspondingly high fatigue strength, although that results in a significant increase in the design effort and manufacturing costs required.
On mobile machines, during the process of starting the internal combustion engine, the hydraulic pump of the working hydraulics (which is located in the drive train) also requires additional energy. Existing electric-motor driven, geared-down starter devices of the internal combustion engine are therefore unsuitable for economically creating a start-stop function having the appropriate fatigue strength and supplying the necessary starting energy.
In drive trains of a similar type used in a mobile machine, hydrostatic power units are used as hydraulic starters of the internal combustion engine for a start-stop function. The hydrostatic power units are in a drive connection with the crankshaft of the internal combustion engine and are operated with hydraulic fluid from a hydraulic accumulator during a starting process of the internal combustion engine. DE 10 2011 105 006 A1 describes a drive train in which, in addition to the hydraulic pump of the working hydraulics, an additional hydraulic motor is provided that functions as a starter of the internal combustion engine to achieve a start-stop function of the internal combustion engine. Because an additional hydraulic motor is in coupled motion during the normal operation of the running internal combustion engine, losses occur which reduce the overall efficiency of the machine.
To eliminate the cost and effort required for an additional hydraulic motor as a starter of the internal combustion engine, the hydraulic pump that is already present for the working hydraulics can be used as a hydraulic starter of the internal combustion engine by operating the hydraulic pump as a motor which is supplied with hydraulic fluid from a hydraulic accumulator on the suction side. A drive train of this type is described in EP 2 308 795 A1. The hydraulic pump, such as an axial piston machine, for example, of the working hydraulics of known drive trains is generally designed for high suction limit speeds (rotational speed) when it is operated as a pump. The suction side with the suction inlet, which in pump operation is connected with the tank, has a correspondingly large cross section. In motor operation, in which the hydraulic accumulator is connected with the suction side of the hydraulic pump of the working hydraulics, the casing of the hydraulic pump generally does not have sufficient strength to withstand high pressures, so that when it is being operated as a motor, the hydraulic pump can be operated only with limited pressures on the suction side. As a result, when the hydraulic pump is operated as a motor, only a limited torque is available in the drive train. To introduce a higher torque into the drive train when the hydraulic pump is operated as a motor, the suction side and the suction inlet of the hydraulic pump could be designed with a high compression strength, although that requires complex and expensive structural modifications to the hydraulic pump which increase the manufacturing costs of the hydraulic pump.
Therefore, it is an object of this invention to provide a drive train of the general type described above but in which an increased torque can be introduced in the drive train with little construction effort or expense.